This invention relates to circuitry for sensing when the operating voltage applied to the circuitry exceeds a predetermined level and for producing a control signal in response to an overvoltage condition.
In many applications, such as, for example, automotive systems, the supply voltage may vary over a wide range. Circuits powered by the supply voltage may be damaged when the supply voltage exceeds a certain overvoltage level (VOV). To prevent the circuits from being damaged, the overvoltage condition must be sensed and power must be removed from the circuits or the circuits must be deactivated.
A known circuit for sensing an overvoltage condition is shown in FIG. 1. The circuit of FIG. 1 includes a PNP transistor, Q1, connected as a diode which is used to prevent current flow between the positive supply line (Vs) and ground when the supply and ground connections are interchanged. A Zener diode, Z1, used to sense the overvoltage condition is connected in series with Q1 and resistors R1 and R2 between Vs and ground. Resistor R1 is used to limit the current which flows through Q1 and Z1 and the value of resistor R2 is selected to ensure the voltage across R2 will be less than 0.5 or 0.6 volts when Z1 is not conducting. An NPN transistor, Q2, whose base-to-emitter junction is connected across R2, is used to control the load circuitry 7 when Z1 breaks down and causes Q2 to conduct.
The operation of the circuit of FIG. 1 may be briefly described as follows:
Assume that Z1 has a breakdown voltage Vz and that Q1 has a forward voltage of Vf. For values of supply voltage (Vs) less than Vz+Vf, there is only leakage current flowing through Q1, Z1, R1 and R2. When Vs exceeds Vz+Vf, a current, Ix, flows through Q1, Z1, R1 and R2. VOV is the value of Vs at which Vs exceeds Vz+Vf and produces a current Ix which causes Q2 to conduct. Transistor Q2 conducts when a voltage drop equal to VBE2 is developed between its base and emitter terminals. The VBE2 drop is produced when the current Ix flowing through Q1, Z1, R1 and R2 reaches a level such that [Ix.multidot.R2] exceeds the VBE of Q2. For values of Vs much less than VOV, the current through Z1 is small (leakage) generating a voltage much less than VBE2 across R2. As Vs increases and approaches VOV, Z1 breaks down and the current through Z1 increases causing the voltage across R2 to rise. When Vs equals VOV, the voltage developed across R2 equals VBE2, current flows into the base of Q2 and the collector current of Q2 is sufficient to turn off (or otherwise deactivate) the load circuitry 7 connected to the collector of Q2.
The circuit of FIG. 1 performs a useful function but suffers from the following disadvantages: 1. When Vs rises to a voltage level where the Zener diode, Z1, just begins to conduct, noise signals may be generated which cause the collector current of Q2 to vary widely. This results in an oscillatory signal being applied to control the load circuitry 7 connected to the collector of Q2. 2. The voltage developed across R2 and the resulting conduction level of Q2 changes as the supply voltage is varied in the vicinity of VOV. If Vs changes gradually, the load circuitry 7 connected to the collector of Q4 will be turned off or on gradually over a range of several millivolts. In this range, noise signals can cause erratic operation of the circuit under control.
These disadvantages are significantly reduced, if not eliminated, in circuits embodying the invention.